storage_file: Declare virStorageSourceParseRBDColonString only in one header

storage_file: Declare virStorageSourceParseRBDColonString only in one header

The virStorageSourceParseRBDColonString() function is declared in
src/storage_file/storage_source.h and
src/storage_file/storage_source_backingstore.h but implemented
only in the .c that corresponds to the latter header file.
Therefore, drop declaration from storage_source.h as the function
is not implemented in its corresponding .c file.

Leftover from: 2d29a3a9d86b5f95a859888283e6caa98593b1d2
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>